The influence of environmental calcium on the branchial morphology in a catadromous fish

Eels are exposed to Ca2+ changes during migration between seawater and freshwater. The gill is the main organ of active calcium transport and has a large surface area to be particularly sensitive to environmental changes in the aquatic environment. In this research, we focused on the morphological changes of gill tissues when eels are faced with the environmental calcium challenges. Based on the results of hematoxylin and eosin (HE) staining and immunohistochemistry, compared with the control group (normal Ca2+ environment), the filament and lamella lengths and lamellar frequency (LF) appeared higher in high calcium environment and lower in deficient calcium environment, while the lamella width and filamental lamellar surface area (SA(FL)) decreased in high calcium environment and increased in deficient calcium environment. And there was no difference in the number filaments in first right gill arch in the three Ca2+ water environment. Transmission electron microscopy was employed to examine the ultrastructural changes in gills in different Ca2+ water environment. The nucleus and endoplasmic reticulum had a tendency to expand in calcium-deficient water, but had a tendency to shrink in high-calcium water comparing with the control group. This study provides the support that branchial surface areas are regulated in different Ca2+ waters through a list of calcium transporters including CACNB2.

Automated summary

This study focused on the morphological changes of gill tissues in eels when faced with environmental calcium challenges, showing differences in filament and lamella lengths, lamellar frequency, and lamella width in different calcium environments. Ultrastructural changes in gills were examined using transmission electron microscopy, indicating expansion in calcium-deficient water and shrinkage in high-calcium water compared to the control group. Branchial surface areas are regulated in different calcium waters through calcium transporters such as CACNB2.